Withdrawal using only L1 transactions

Users should be able to withdraw from the subnet contract using only L1 transactions. This is important because it protects the user from the miner disappearing. Currently if the subnet chain is not progressing, then assets can't be withdrawn and they are stuck in the contract forever. Instead, new L1 Ops should be added that are able to trigger the withdrawal. This would also simplify the normal process, so instead of a 2 step withdrawal, with one tx on the L2 and one on the L1, there could be just one tx on the L1. This will require a bit of a change from the current implementation, but I think it would be a good idea to re-think.

This is not an urgent task, but just to remember to think more about this for a later iteration.